An automobile automatic transmission utilizes a hydraulic governor to provide a varying pressure proportional to the speed of the output shaft of the transmission. There is also a mechanism in the transmission to provide a hydraulic pressure proportional to the throttle position, referred to as throttle pressure. The shift points are determined by a combination of the governor pressure and the throttle pressure. An electronic closed-loop system comprising a variable force solenoid and a hydraulic pressure sensor is used to control the governor pressure. The electronics that provide this function are contained in the powertrain control module (PCM). FIG. 1 illustrates a conventional PCM 10 coupled to various sensors. In an example, the Chrysler 47RE & 48RE series of transmissions require electronic control of a hydraulic governor 12 that utilizes a variable-force solenoid 14 and a pressure sensor 16 to provide feedback. The shift points are determined by the throttle pressure and governor hydraulic pressure which is controlled by the PCM 10. The PCM 10 applies an algorithm that causes the governor pressure to vary from an otherwise linear relationship to vehicle speed based upon the vehicle speed sensor 20 and throttle position sensor 22. There are several curves employed in the PCM 10 for use in different climatic and operational environments.
It is common practice to add aftermarket engine performance enhancements to vehicles. Relatively small torque increases (100+ ft. lbs.) can cause torque converter clutch slippage, particularly at part throttle. A common technique to overcome this problem is to adjust the hydraulic line pressure 18 (on the order of +8-10 psi) in the transmission. This adjustment is provided by an adjustment screw on the valve body of the transmission. This additional pressure in turn allows proportionally higher hydraulic pressure on the torque converter clutch and the various clutches and bands in the transmission, thus mitigating or eliminating minor slippage.
Modification of the line pressure beyond the manufacturer's specification also causes side effects, including, but not necessarily limited to:
1. Loss of full-throttle first-to-second gear upshift;
2. Vacillation between first and second gear at low speed and light throttle application;
3. Early upshifts; and
4. No 2-1 downshift resulting in second gear starts.
5. PCM error codes reported due to the line pressure being out of a predetermined range.
Presumably, the calibration of the governor pressure vs. throttle pressure is thrown off, causing some or all the aforementioned side effects.
The governor pressure is balanced against throttle pressure which determines the upshift point. The effect of raised line pressure at some point reverses the lower shift point side effect to a higher shift point side effect. This is evident by the fact that no 1-2 upshift occurs at full throttle. The actual cause has not been extensively investigated, but presumably, it is due to the governor pressure not reaching the point at fill throttle to cause a 1-2 upshift. A common solution to these problems is to add a resistor or voltage regulator in series with the sensor excitation voltage lead 24 from the PCM 10 to the transmission. This practice is not without its own side effects, though. While curing some of the side effects, other shift points are earlier as a result of the higher governor pressure, which is generally undesirable.
Accordingly, what is needed is a system and method that allows the use of increased line pressure to overcome the problem of clutch slipping while minimizing the above-identified problems with conventional solutions. The present invention addresses such a need.